1. Field of Invention
The present invention relates to a method of manufacturing an exposure mask having an unprecedented supermicrostructure for carrying out an X-ray exposure method favorable for conventional supermicro exposure. The method employs lithography techniques to achieve microminiaturization resulting in better performance of high density integrated circuits and transistors for use in the electronics field.
2. Description of the Related Art
X-ray exposure lithography shows promise for providing a process that will achieve a breakthrough beyond the limits of present optical lithography techniques, as shown in FIGS. 1A and 1B.
However, X-ray exposure methods are proving difficult to implement. As a mask for preventing X-rays, a fine pattern of metal such as gold, tungsten and the like must be formed on a thin film. At present, an electron beam exposure method is being widely used as a technique which is not mass-producible but can draw a comparatively fine pattern. Consequently, frequent attempts have been made to form a fine metal pattern in combination with this exposure method using etching, lift-off and plating methods. However, it is very difficult to form a superfine metal pattern having a sufficient thickness by scattering electron beams in a resist or in structures under a resist such as a metal substrate or the like. This problem of supermicrostructure in electron beam exposure methods, limits supermicro-structures to about 10 nm of frequency at present.
On the other hand, in the formation of a layer structure, it is easy to provide film thickness controllability of about 1 nm. Oblique deposition is one example of a mask using the layer structure (see Appln. Phys. Lett., 36 (1), 1 Jan. 1980, American Institute of Physics). According to this method, as shown in FIGS. 1A and lB, a mask having about a 2 nm minimum ray width is formed on a periodically square-wave polyimide membrane by oblique deposition of carbon and tungsten, and a fine ray structure of up to 17.5 nm. However, the use of an oblique deposition method cannot minimize the period of a square wave.
At present, a thin film crystal growth method is the most closely studied method of forming a layer structure, and particularly in compound semiconductor crystal growth, composition switching controllability of less than one atomic layer is possible. At the same time, it is possible to grow crystal on the whole side surface of a deep groove. These advantages are due to crystal growth under such condition that migration of a growing seed on the crystal surface is selected to be large thereby maintaining flatness. Moreover, if thin film crystal growth is carried out by using single atomic layer epitaxial techniques, it is possible to form a thin film to within one atomic layer interval on the whole growth surface.
X-ray exposure is a method of going beyond the limits of optical wavelengths in the conventional exposure techniques. However, the difficulty of manufacturing an exposure mask has emerged as a new problem. That is, it is very difficult to form a very fine mask which can prevent penetration by X-rays on the order of 10 nm as required for an X-ray mask.